1. Field of the Invention
The present invention relates to a toner supplying device for supplying a toner to a developing device of an image forming apparatus, and to an image forming apparatus having the toner supplying device.
2. Description of the Related Art
Electrophotographic image forming apparatuses such as copiers and laser printers form an image by performing the following processes:
(1) forming an electrostatic latent image on an image bearing member;
(2) developing the electrostatic latent image using a developing device to form an unfixed toner image on the image bearing member;
(3) transferring the unfixed toner image onto a recording material (such as paper sheets); and
(4) fixing the toner image to the recording material using a fixing device.
A key component is the developing device, and a toner supplying device for supplying a toner to the developing device is typically provided in such image forming apparatuses.
As one example of the toner supplying device, there is a toner supplying device having a sub-hopper, which is provided between a toner container and a developing device to temporarily contain a toner to be supplied to the developing device. The sub-hopper has an internal partition for separating the sub-hopper vertically into upper and lower chambers, a powder pump for transporting the toner from the toner container to the upper chamber of the sub-hopper, a toner circulating device (i.e., an upper screw) for circulating the toner in the upper chamber, and a lower screw provided in the lower chamber for supplying the toner to the developing device. Further, the toner supplying device has a rib for preventing occurrence of a toner influx in which the toner, which is transported from the toner container to the upper chamber, is fed to the developing device without being circulated in the upper chamber, and a controller for controlling supply of the toner such that, when the powder pump is operated to transport the toner from the toner container to the sub-hopper, the lower screw is not operated to prevent occurrence of the toner influx.
Specifically, the mechanism of the toner influx occurring in such a toner supplying device is an increase in pressure in the sub-hopper as the toner is supplied from the toner container by the powder pump, particularly when the upper and lower screws are operated while the powder pump is operated, thereby feeding excess amounts of toner to the developing device. By performing control such that the toner feeding operation performed by the upper and lower screws in the sub-hopper is not performed when the powder pump is operated, increase in pressure of the sub-hopper is avoided, thereby preventing occurrence of the toner influx.
As another example of the toner supplying device, a toner supplying device controlling the concentration of toner in a developer by estimating the toner concentration based on the toner concentration measured on an upstream side therefrom relative to the developer circulating direction is known. Specifically, in the toner supplying device, a toner supplying member is driven by a driving source to feed a toner supplied from a predetermined supply point to a developing roller while measuring concentration of the toner in a developer passing through a first point upstream from the supply point relative to the developer circulating direction. In addition, change of the toner concentration in the developer passing a second point located downstream from the supply point and upstream from the developing roller is estimated to control the toner concentration by controlling power supply from the driving source on the order of several hundreds of milliseconds based on the estimated toner concentration.
On the other hand, image forming apparatuses are often needed to perform a repeat imaging operation in which the same image having a high image area ratio is repeatedly formed continuously on consecutive recording sheets. In this case, a toner supplying operation (i.e., a feeding screw driving operation) has to be performed on a high duty. In this regard, it is likely that the image density of produced images decrease if toner supply control is not satisfactorily performed.
For these reasons, there is a need for a toner supplying device which can reliably supply toner to a developing device without causing the toner influx and without forming low-density images in a repeat imaging operation.